don't raise exceptions splitting a blank string

Author: Montana Low

pandas/core/strings.py

@@ -1633,7 +1633,8 @@ def cons_row(x):
             if result:
                 # propagate nan values to match longest sequence (GH 18450)
                 max_len = max(len(x) for x in result)
-                result = [x * max_len if x[0] is np.nan else x for x in result]
+                result = [x * max_len if len(x) == 0 or x[0] is np.nan
+                          else x for x in result]
 
         if not isinstance(expand, bool):
             raise ValueError("expand must be True or False")

pandas/tests/test_strings.py

@@ -1942,6 +1942,13 @@ def test_split(self):
         exp = Series([[u('a'), u('b'), u('c')], [u('c'), u('d'), u('e')], NA,
                       [u('f'), u('g'), u('h')]])
         tm.assert_series_equal(result, exp)
+        
+        # expand blank split
+        values = Series(['a b c', 'a b', '', ' '])
+        result = values.str.split(expand=True)
+        exp = DataFrame([['a', 'b', 'c'], ['a', 'b', np.nan],
+                         [np.nan, np.nan, np.nan], [np.nan, np.nan, np.nan]])
+        tm.assert_series_equal(result, exp)
 
     def test_rsplit(self):
         values = Series(['a_b_c', 'c_d_e', NA, 'f_g_h'])